Hide Brave Rewards card (during brave://welcome)

[bsclifton]

Test plan

1. Fresh profile
2. Launch Brave
3. brave://welcome screen will show
4. Hit next until you get to end
5. Brave Rewards card should NOT show

Description

Desktop onboarding shows brave://welcome which acts as a welcome tour. It has an intro card, import, shields and P3A, and lastly Brave Rewards. We should tie the the Brave Rewards card to a configuration value so that we can easily turn on and off

Let's default this to off to start

[bsclifton]

This might not be possible to take effect on first launch; the value won't be fetched from variation service until 2nd launch

https://github.com/brave/brave-variations#some-notes-on-using-variations-in-the-browser

Studies only take effect after restarting the browser.

[rebron]

Related issue: #21202

[GeetaSarvadnya]

Verification PASSED on

Brave | 1.36.115 Chromium: 99.0.4844.51 (Official Build) (64-bit)
-- | --
Revision | d537ec02474b5afe23684e7963d538896c63ac77-refs/branch-heads/4844@{#875}
OS | Windows 10 Version 21H2 (Build 19044.1586)

• Verified the description from the issue and confirmed Brave Rewards card is removed from the welcome onboarding flow in both clean and upgraded profile

Clean profile

Example	Example	Example	Example

Upgrade profile

1.36.112

Example	Example	Example	Example	Example

After upgrade to 1.36.115

Example	Example	Example	Example

---

Verified using

Brave | 1.36.115 Chromium: 99.0.4844.51 (Official Build) (x86_64)
-- | --
Revision | d537ec02474b5afe23684e7963d538896c63ac77-refs/branch-heads/4844@{#875}
OS | macOS Version 12.2.1 (Build 21D62)

Verified test plan from #21400 (comment)

• Confirmed rewards card is no longer visible from brave://welcome

First	Second	Final

---

Verification passed on

Brave	1.36.115 Chromium: 99.0.4844.51 (Official Build) (64-bit)
Revision	d537ec02474b5afe23684e7963d538896c63ac77-refs/branch-heads/4844@{#875}
OS	Ubuntu 18.04 LTS

• Confirmed rewards card is no longer visible from brave://welcome